One conventional method of producing ground wood pulp for the manufacture of paper products involves pressing a batch of pulpwood (roundwood or wood chips) against a rotating grinding stone while simultaneously feeding shower water into the grinding chamber, specifically by spraying the water directly on the surface of the stone at a location spaced from the actual grinding location. By means of a dam or weir, the formed groundwood stock, which is an aqueous slurry of pulp, is kept in the grinding chamber at a level a little higher than the lower point of the stone in order to clean, lubricate and cool the stone. The ground wood stock flowing over the dam is discharged by its own weight for further treatment. A variant of the foregoing is the "pitless" method, in which the stone is not immersed, and provision is made for extra water showers.
Another known method utilizes a disc refiner, in which material being refined or reduced is worked between two closely spaced opposed discs which undergo relative rotation.
In a recent development, the wood is ground under superatmospheric pressure, thus permitting grinding temperatures higher than in the standard stone groundwood (SWG). In U.S. Pat. Nos. 3,808,090 and 3,948,449, a process is described for improving the groundwood pulp by grinding wood in a closed grinding chamber in a pressurized gaseous atmosphere. In the two patents just named, the wood is fed in and the superatmospheric pressure in the grinding chamber can be maintained only so long as the grinding of a wood batch continues. However, when a new wood batch must be fed into the magazine, the magazine must be opened and the pressure of the grinding surface falls to atmospheric. Thus, the grinder does not work in a continuously pressurized atmosphere.
In an attempt to overcome the problem just defined, additional developments have been made and patented by Oy Tampella Ab, as exemplified in Canadian Pat. No. 1,097,118 issued Mar. 10, 1981, and U.S. Pat. Nos. 4,270,703 and 4,274,600 issued June 2, 1981 and June 23, 1981, respectively. In the Oy Tampella process, a feed chamber upstream of the grinding chamber has two pressure seals, one to the atmosphere and one to the grinding chamber. Thus, the feed chamber acts as a double-lock seal, to allow the pressure in the grinding chamber always to be maintained above atmospheric. By the use of this method, the pressure in the grinding chamber may reach as high as several bar, and temperatures at the grinding stone surface may climb well above the standard pressure boiling point.
Because of the considerable size and complexity of the SGW process and the pressurized groundwood (PGW) process developed hitherto, it would be desirable to reduce the complexity and size of an installation for producing ground pulp that can be used in paper making. In both the PGW and SGW processes, very large pressure shoes must be hydraulically driven to urge the roundwood against the grinding stone, and above the general location of the pressure shoes must be provided a stack for the incoming wood to be ground.
A different approach to the grinding of wood pulp is one in which the grinding pressure between the wood and the grinding surface is brought about centrifugally, by providing an internal cylindrical grinding surface, and by "flinging" the wood outwardly against the stationary grinding surface through the use of centrifugal force. The centrifuging action not only would allow the appropriate pressure to arise between the wood and the grinding surface, but could also pressurize a quantity of water being swept around along with the wood, thus permitting higher temperatures than the maximum attainable in the standard SGW process.
Early Canadian Pat. No. 2835, issued Oct. 24, 1873 to Moore, discloses a primitive version of a centrifugal grinder for wood, which incorporates a stationary internal cylindrical grinding surface, and a rotor turning about a vertical axis, and flinging the feed wood centrifugally outward along radial pathways to contact the grinding surface. Water for cooling the grinding surface and for making up the pulp slurry is simply squirted into the housing by a single hose or pipe.
Because of the primitive construction utilized by Moore, his apparatus would not do for the high speed grinding requirements of the present day.
In my earlier U.S. Pat. No. 4,474,335, issued on Oct. 2, 1984 and entitled "Method and Apparatus for Centrifugal Pulpwood and Wood Chip Grinding", there is described a grinding apparatus which utilizes the rotating principle in order to promote uniform and pressurized water spray against the internal grinding surface. My said earlier patent describes a centrifugal grinder which includes an internal grinding surface in the shape of a surface of revolution, a rotor mounted for rotation coaxially within the grinding surface, the rotor having a central cavity and defining at least one pocket through which material in the central cavity can contact the grinding surface, first means for delivering material to be ground to the central cavity, second means for rotating the rotor and third means in the rotor for applying water to the grinding surface.
While the apparatus described in my earlier patent functions quite satisfactorily, on-going tests have shown the propensity of fibre to stick to the inside surface of the internal pulpstone grinding surface, despite provisions designed to minimize this occurrence.
Accordingly, it is an aim of one aspect of this invention to provide an improved construction for a centrifugal grinder, in which the pulp slurry can be lifted from the stone surface and then caught or collected in a special pocket-defining unit, from which it can be passed to a compartment axially beyond the grinding surface.
More particularly, this invention provides, in combination:
a grinding stone having an internal grinding surface in the shape of a surface of revolution, PA1 a rotor within the grinding stone mounted for rotation about the axis of the surface of revolution, the rotor having an axial inlet for material to be ground and pocket means to allow the material to contact the said internal grinding surface, PA1 spray means mounted on the rotor at a location trailing the said pocket means in the sense of rotation, the spray means adapted to spray liquid against the internal grinding surface to make a slurry with ground material, PA1 an element mounted on the rotor at a location trailing the said spray means in the sense of rotation, the element having a rounded outer surface juxtaposed closely with respect to the said internal grinding surface, PA1 a scoop means mounted on the rotor and trailing the said element in the sense of rotation, the scoop means having a leading edge juxtaposed against the said internal grinding surface and defining a pocket for collecting the scooped-up slurry, the scoop means being adapted to conduct the slurry toward the axial ends of the grinding stone, PA1 means defining an outlet for the slurry, PA1 and passage means from the pocket of the scoop means to the said outlet. PA1 (a) rotating the rotor while feeding into the rotor material to be ground, and spraying liquid onto the internal grinding surface to make a slurry with the ground material, PA1 (b) at a location trailing the spray means in the sense of rotation, closely juxtaposing with respect to the internal grinding surface an element having a rounded outer surface, thereby to lift the slurry inwardly away from the internal grinding surface, PA1 (c) at a location trailing the element in the sense of rotation, scooping the slurry in a scoop means having a leading edge closely juxtaposed with respect to the internal grinding surface, PA1 (d) and transferring the scooped-up slurry to a compartment located axially beyond the grinding surface.
Further, this invention provides a method of grinding a material utilizing the combination of a grinding stone having an internal grinding surface in the shape of a surface of revolution and a rotor within the grinding stone mounted for rotation about the axis of the surface of revolution, the rotor having an axial inlet for material to be ground and pocket means to allow the material to contact the said internal grinding surface, and spray means on the rotor for spraying liquid against the internal grinding surface, the method comprising the steps: